We will continue to explore the energetics of the interaction of antigens with immunocompetent cells but in a new direction. Thus, we will study the possibility that cell surface IgM and IgD have different binding characteristics for antigen. We also are interested in the secondary effects of this binding such as capping, endocytosis and regeneration of the receptor, as possible mechanisms by which these 2 isotypes confer different reactivity on the cells on which they reside. We have recently obtained an antiserum to murine delta chain and will use this antiserum in blocking, killing and cell sorting experiments to determine how IgD regulates the immune response. The in vitro model of tolerance induction will be pursued in order to determine the underlying differences between the surface molecules and/or metabolism of neonatal versus adult cells that account for the marked susceptibility of the neonatal cells to tolerance induction by antigen. BIBLIOGRAPHIC REFERENCES: Melcher, U. and J. W. Uhr. Cell surface immunoglobulin XVI. Polypeptide chain structure of mouse IgM and IgD-like molecule. l976. Jour. of Immunol. 116, 409. Vitetta, E.S., and J.W. Uhr. Cell surface immunoglobulin XV. The presence of IgM and an IgD-like molecule of the same cell in murine lymphoid tissue. 1976. Eur. J. Immunol. 6, 140.